For some fiber optic applications, including the areas of fiber amplifiers and lasers, optical fibers may be coupled to core-less endcaps. In some examples, these endcaps can be formed as homogeneous transparent elements with a length of a few millimeters. Since the endcaps contain no waveguide (fiber core), light propagates in these regions as beams that expand toward the ends of the endcaps.
Endcaps can be formed by attaching (e.g. fusion splicing) small pieces of homogeneous glass to the fiber ends. In some situations, such as photonic crystal fibers, it can be sufficient to heat the fiber end (e.g. with a fusion splicer) to collapse microscopic holes.
A fiber laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium or holmium. Fiber lasers are related to doped fiber amplifiers, which provide light amplification without resonant oscillation. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.